The present invention relates generally to voltage controllers for power systems, and more particularly to a power system voltage controller used at a load-site.
Electrical power generation is often performed at generation sites distant from the consumers of electrical power. The electrical power is transmitted from the generation sites to the consumers by feeder distribution networks. Local electrical generator units are sometimes used at load sites to augment utility supplied electrical power. Local electrical generators may be used to provide electrical power at a cost cheaper than that supplied by the utility. Large consumers of electricity, such as manufacturing plants and the like, sometimes find it economical to produce some or all of the electrical power they require from local electrical generators, such as micro turbines, rather than to purchase the power from a utility.
For ideal economic local generation of power, the local electrical generator operates at capacity, and all of the power generated by the local electrical generator is used by the load. Further, it is desirable to limit the real power supplied by the utility in some locations to zero. In addition, the reactive capability of the local micro turbine power generation system can be used to control the local voltage, especially at the end of high impedance feeders.
Control of the inverter to achieve this is difficult, however. Due to changes in the load the requirements placed on the inverter to operate micro turbines and maintain efficiency may change dynamically, both as to real and reactive power requirements.
A simplified single line diagram of a local electrical generator coupled to a load in parallel with a feeder distribution system is illustrated in FIG. 1. Although FIG. 1 includes features of the present invention, FIG. 1 is also useful in describing the background of the present invention. As illustrated in FIG. 1, a utility 11 is connected via a source impedance 13 to a local power generation and distribution system 15. The local power generation and distribution system comprises a local energy source 19, whose output power is modified to be compatible with the utility voltage and frequency by an inverter 21, coupled to a load 17. The inverter is, for example, a pulse width modulated (PWM) inverter.
An output filter 23 reduces the harmonic content produced by the inverter. The filter is connected to the load and utility by an additional impedance 25 of the distribution system. The inverter is controlled by a control system 27 that senses voltages and current and regulates the inverter to perform required functioning.
An example power regulator system is illustrated in FIG. 2. In the system of FIG. 2, a local power source and associated inverter (indicated together) 351 are coupled to a transmission line 325 at a load site. The power source and associated inverter provide power to a load 327. Coupled to the connection between the power source and associate inverter and the load is a filter including a capacitor 329. The local power source is therefore connected in parallel to the utility.
The power regulator system of FIG. 2 includes an inverter current regulator (311 and 323). The current regulator provides a signal to the local power source and associated inverter for use in the control of the power source and associated inverter. In the system illustrated in FIG. 2, an inverter current output vector i.sub.inv of the inverter is regulated to a desired value.
The current regulator is a vector control system based upon a Park-vector, or space-vector, representation of all three-phase electrical quantities. The use of Park-vectors facilitate transformation of control signals from sinusoidal values in a stationary frame to largely DC level signals in a synchronous frame. Methods of transforming signals from one reference frame to another is well known by those familiar with the art. Park vectors are described in, for example, Transient Phenomena in Electrical Machines by P.K. Kovacs, published by Elsevier (1984), the disclosure of which is incorporated herein by reference.
Accordingly, the inverter current output vector i.sub.inv is determined. As the inverter current output vector i.sub.inv is measured in the stationary reference frame, a capacitor voltage vector v.sub.cap is also determined for use in transforming the inverter current output vector to the synchronous frame. In order to reduce ac signal components in the synchronous frame signal, the capacitor voltage is filtered to reduce harmonics and other noise at frequencies other than those about the fundamental system frequency. Therefore, a rotational reference frame is extracted from the filtered capacitor voltage vector to form a unit vector for transformation to the synchronous frame in an extraction unit 331. The unit vector is provided to a transformation unit 332, as is the inverter current output vector i.sub.inv. The transformation unit 332 outputs a vector i.sub.k, which is comprised of essentially DC signals of a real component and a reactive component, representing the inverter current vector in the synchronous frame. The vector i.sub.k, therefore, is the inverter current output vector in the synchronous frame.
The vector i.sub.k is compared with a command reference vector i.sub.ikcmd at a summer 323. Generally the command reference signal i.sub.ikcmd is empirically determined, and is changed only infrequently. As it is often desirable to provide as much real power from a local power source generator to the load as possible, the real power component is generally set to a maximum, which is a value of one power unit (p.u.) in a normalized system. The reactive component of the command reference signal i.sub.ikcmd is generally set to 0.
The output of the summer 323 is provided to a controller 311. The controller 311, in the prior art, amplifies the output of the summer, and provides a voltage vector command v.sub.ik in the synchronous frame. The voltage vector command provided by the controller is transformed to the stationary frame by a transformation unit 333, again based upon a unit vector provided by the extraction unit 331. The output 313 of the transformation unit is provided to the local power source and associated inverter 351 to control inverter operation.
The control system of FIG. 2, as described above, is well known to those skilled in the art. Such a control system supplies constant real power to the utility. The system of FIG. 2, however, does not optimize provision of reactive power to the system, and does not adaptively modify local power supply output based on changes in real power requirements. Further, in the system of FIG. 2 the filter may introduce unwanted power variation, particularly about resonant frequencies of the capacitor.